Like everyone else said, when they tell you. The first school horse I rode and took lessons on was named dusty, haha I've met adults that learned to ride on him. He was going strong for about 25 years. I find the time you stop riding a horse is the time they die, even if you at least just hop on bareback (or put a small kid on them) it keeps them going and going and going... haha I think they need a perpose, rather then to just stand in the feild, any old horse I've known has died almost right after they had stopped being used for anything.
My horse is 20 and i'm jumping with him like he did it ten years ago...
He's a good teacher with a lot of experience in the big show jumping. He likes jumping and I learn a lot from him and he has already the experience I need!
He jumps over 1,20 cm with me, whether at shows or at home. He's born for the sport and this horse would be unhappy if he wouldn't be allowed to jump!